Gas sensor control device

ABSTRACT

An O 2  sensor includes a sensor element using a solid electrolyte layer and a pair of electrodes placed at a position to interpose the solid electrolyte layer, detects an exhaust gas from an internal combustion engine as an object of a detection, and outputs an electromotive force signal depending on an air-fuel ratio of the exhaust gas. The sensor element is connected with a constant current circuit supplying a constant current that is prescribed. A microcomputer calculates an element resistance, determines whether the air-fuel ratio is at least rich, lean, or stoichiometric, on the basis of a comparison between an electromotive force output of the electrogenic cell and a prescribed threshold. Further, the microcomputer controls the constant current supplied by the constant current circuit on the basis of the element resistance.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2013-202133filed on Sep. 27, 2013, the disclosure of which is incorporated hereinby reference.

TECHNICAL FIELD

The present disclosure relates to a gas sensor control device.

BACKGROUND ART

For example, a vehicle engine generally uses an electromotive forceoutput type gas sensor which takes the exhaust gas discharged from theengine as an object of detection and detects the oxygen concentration.The gas sensor has an electrogenic cell which outputs an electromotiveforce signal which differs depending on whether the exhaust gas air-fuelratio is rich or lean. Specifically, when the air-fuel ratio is rich,the gas sensor outputs an electromotive force signal of about 0.9 V andwhen the air-fuel ratio is lean, the gas sensor outputs an electromotiveforce signal of about 0 V.

As for this kind of gas sensor, attention has been drawn to the factthat when the air-fuel ratio of the exhaust gas changes to rich or lean,the sensor output changes with a delay from the actual change of theair-fuel ratio. Various techniques have been described to improve thisoutput characteristic.

For example, in the gas sensor control device in Patent Literature 1, aconstant current circuit is connected to at least one of a pair ofsensor electrodes. When it is determined that a change request to changethe output characteristic of the gas sensor has been generated, thedirection of constant current is determined according to the changerequest and the constant current circuit is controlled so that theconstant current flows in the determined direction. Thus, the outputcharacteristic of the gas sensor is appropriately controlled bysupplying the constant current.

In a gas sensor, the resistance value of the sensor element changesdepending on the temperature of the sensor element. Specifically, whenthe engine is started in the cold or when the exhaust gas temperaturedecreases with fuel cut to the engine, the element resistance increaseswith the decrease in the temperature of the sensor element. In thiscase, as the element resistance increases, the voltage applied to thesensor element increases even under the condition that a constantcurrent flows. Because of this, the accuracy of determination about arich or lean air-fuel ratio may decrease. The sensor element correspondsto an electrogenic cell. The resistance value of the sensor element isalso called the element resistance.

More specifically, in the microcomputer which receives electromotiveforce output from the gas sensor, a first threshold which is on a richerside than the stoichiometric value (0.45 V) and a second threshold whichis on a leaner side than the stoichiometric value (0.45 V) arepredetermined. For example, the first threshold is set to 0.6 V and thesecond threshold is set to 0.3 V. When the electromotive force output islarger than the first threshold, the air-fuel ratio is determined asrich and when the electromotive force output is smaller than the secondthreshold, the air-fuel ratio is determined as lean. In this case, whenthe voltage applied to the sensor increases due to an unintentionalchange in the element resistance, air-fuel ratios which are determinedas rich (or lean) would vary widely, resulting in decrease in theaccuracy of air-fuel ratio determination. This problem arises becausethe air fuel ratio is determined as rich or lean not in the region ofthe gas sensor output characteristic where the electromotive forceoutput rapidly changes, but its stable region which is on a richer orleaner side than the rapid change region. Therefore, in ensuring theaccuracy of air-fuel ratio determination, there is room for improvement.

PRIOR ART LITERATURES Patent Literature

Patent Literature 1: JP2012-63345A

SUMMARY OF INVENTION

The present disclosure has an object to provide a gas sensor controldevice which can make an air-fuel ratio determination appropriatelyunder a condition that a constant current is supplied to a gas sensor.

According to an aspect of the present disclosure, a gas sensor controldevice is applied to a gas sensor which has an electrogenic cell using asolid electrolyte body and a pair of electrodes placed at a position tointerpose the solid electrolyte body, and detects an exhaust gas from aninternal combustion engine as an object of a detection and outputs anelectromotive force signal depending on an air-fuel ratio of the exhaustgas The gas sensor control device includes a constant current supplyingsection supplying a constant current that is prescribed to theelectrogenic cell, a resistance value calculating section calculating aresistance value of the electrogenic cell, an air-fuel ratio determiningsection determining whether the air-fuel ratio is at least rich, lean,or stoichiometric, on the basis of a comparison between an electromotiveforce output of the electrogenic cell and a prescribed threshold, and acurrent control section controlling the constant current supplied by theconstant current supplying section, on the basis of the resistance valueof the electrogenic cell calculated by the resistance value calculatingsection.

While a constant current is supplied to an electrogenic cell, the outputcharacteristic of the electrogenic cell shifts to either the rich sideor lean side. At this time, in connection with the electromotive forceof the sensor element, a voltage change equivalent to “elementresistance (resistance value of the electrogenic cell)×constant current”occurs and when the output characteristic shifts to the rich side, avoltage change equivalent to “element resistance×constant current”occurs on the negative side or when the output characteristic shifts tothe lean side, a voltage change equivalent to “elementresistance×constant current” occurs on the positive side. In such case,the amount of voltage change to become larger than expected when due toan unintentional change in the element resistance, the accuracy ofair-fuel ratio determination decreases.

According to the above configuration, since the constant currentsupplied to the electrogenic cell is controlled according to theresistance value of the electrogenic cell, the disadvantage that theaccuracy of air-fuel ratio determination decreases unintentionally canbe suppressed. In other words, the increase in the amount of change inthe sensor applied voltage due to increase in the resistance value ofthe electrogenic cell can be suppressed by decreasing the constantcurrent. Thus, the decrease in the accuracy of air-fuel ratiodetermination can be suppressed. Consequently, air-fuel ratiodetermination can be made appropriately while the constant current issupplied to the gas sensor.

BRIEF DESCRIPTION OF DRAWINGS

The above and other objects, features and advantages of the presentdisclosure will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings:

FIG. 1 is a schematic diagram which shows the general configuration ofan engine control system;

FIG. 2 is a diagram which shows the cross-sectional structure of asensor element and the general structure of a sensor control section;

FIG. 3 is an electromotive force characteristic graph which shows therelation between excess air ratio and the electromotive force of thesensor element;

FIG. 4 is a schematic diagram which shows the reaction of gas componentsin the sensor element;

FIG. 5 is an electromotive force characteristic graph which shows therelation between excess air ratio and the electromotive force of thesensor element;

FIG. 6 is a diagram which shows the structure of the sensor controlsection;

FIG. 7 is an electromotive force characteristic graph which shows therelation between electromotive force output and the supply of constantcurrent;

FIG. 8 is a flowchart which shows the constant current control process;

FIG. 9 is a flowchart which shows the element resistance calculatingprocess;

FIG. 10 is a graph which shows the relation between element resistanceand current correction value;

FIG. 11 is a flowchart which shows the constant current control process;and

FIG. 12 is a graph which shows the relation between element resistancedeviation and current correction value.

DESCRIPTION OF EMBODIMENTS

Next, an embodiment of a gas sensor control device according to thepresent disclosure will be described referring to drawings. Thisembodiment concerns an engine control system which uses a gas sensorlocated on the exhaust pipe of an on-vehicle engine (internal combustionengine) to perform various controls, etc. of the engine according tooutput of the gas sensor. The control system, centered on an electroniccontrol unit (ECU), performs control of the amount of fuel injection,control of ignition timing and so on. FIG. 1 is a block diagram whichshows the general configuration of the system.

In FIG. 1, an engine 10 is, for example, a gasoline engine whichincludes a throttle valve 11 that is electronically controlled, a fuelinjection valve 12, and an ignition device 13. An exhaust pipe 14 of theengine 10 is provided with catalysts 15 a and 15 b as exhaust gaspurifying devices. The exhaust pipe 14 corresponds to an exhaustsection. The catalysts 15 a and 15 b are, for example, both three-waycatalysts; the catalyst 15 a is a first catalyst as an upstream catalystand the catalyst 15 b is a second catalyst as a downstream catalyst. Aswidely known, a three-way catalyst purifies three major emission toxiccomponents, carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide(NOx) such as NO, and is structured so that metal such as platinum,palladium, or rhodium is supported by a honeycomb or lattice-shapedceramic support. In this case, the three-way catalyst purifies CO and HCas rich components by oxidation action and NOx as a lean component byreduction action.

An A/F sensor 16 is located upstream of the first catalyst 15 a and anO₂ sensor 17 is located between the catalysts 15 a and 15 b (downstreamof the first catalyst 15 a and upstream of the second catalyst 15 b).The A/F sensor 16 outputs an A/F signal which is roughly proportional tothe air-fuel ratio of the exhaust gas. The O₂ sensor 17 also outputs anelectromotive force signal which differs depending on whether theair-fuel ratio of the exhaust gas is lean or rich.

The system further includes various sensors including a throttle openingsensor 21 which detects the opening of the throttle valve 11, a crankangle sensor 22 which outputs a rectangular crank angle signal at everyprescribed crank angle of the engine, an air quantity sensor 23 whichdetects the quantity of intake air in the engine 10, and a cooling watertemperature sensor 24 which detects the temperature of engine coolingwater. In addition to the above, the system includes a combustionpressure sensor which detects the combustion pressure in the cylinder,an accelerator opening sensor which detects the opening of theaccelerator (amount of operation of the accelerator), and an oiltemperature sensor which detects the temperature of engine lubricant,though not shown in the figure. In this embodiment, the prescribed crankangle is 30° CA cycle. These sensors correspond to an operationcondition detecting section.

An ECU 25 is mainly comprised of a known microcomputer 41 which includesa CPU, ROM, and RAM, and executes various control programs stored in theROM to perform various controls of the engine 10 depending on eachengine operation condition. In other words, the ECU 25 receives signalsfrom the above various sensors, etc. and calculates the amount of fuelinjection and ignition timing according to the various signals tocontrol the drive of the fuel injection valve 12 and the ignition device13.

In connection with the amount control of fuel injection, the ECU 25performs air-fuel ratio feedback control according to a detection signalfrom the A/F sensor 16 on the upstream of the first catalyst and adetection signal from the O₂ sensor 17 on the downstream of the firstcatalyst. Specifically, the ECU 25 performs main feedback control sothat the actual air-fuel ratio (actual air-fuel ratio on the catalystupstream side) detected by the A/F sensor 16 becomes a target air-fuelratio set according to the engine operation condition, and also performssub-feedback control so that the actual air-fuel ratio (actual air-fuelratio on the catalyst downstream side) detected by the O₂ sensor 17becomes the target air-fuel ratio. In sub-feedback control, for example,according to the difference between the actual air-fuel ratio on thecatalyst downstream side and the target air-fuel ratio, the target airfuel ratio in main feedback control is modified or the amount offeedback correction in the main feedback control is modified. Forair-fuel ratio control, for example, the ECU 25 performs stoichiometricfeedback to make the target air-fuel ratio stoichiometric or nearlystoichiometric. In this case, stoichiometry is equivalent to atheoretical air-fuel ratio.

Next, the structure of the O₂ sensor 17 on the downstream of the firstcatalyst will be described. The O₂ sensor 17 has a sensor element 31with a cup-shaped structure. FIG. 2 shows the cross-sectional structureof the sensor element 31. Specifically, the sensor element 31 has aroughly U-shaped cross section. Actually, the sensor element 31 isentirely housed in a housing or element cover and installed in theengine exhaust pipe. The sensor element 31 corresponds to anelectrogenic cell.

The sensor element 31 has a solid electrolyte layer 32 with a roughlyU-shaped cross section and an exhaust side electrode 33 on its outersurface and an air side electrode 34 on its inner surface. Theseelectrodes 33 and 34 lie as layers on the surfaces of the solidelectrolyte layer 32. The solid electrolyte layer 32 has an oxygenion-conductive sintered oxide made by dissolving CaO, MgO, Y2O3, Yb2O3or the like as a stabilizer in ZrO₂, HfO₂, ThO₂, Bi2O3 or the like. Theelectrodes 33 and 34 are both made of a catalytically active preciousmetal such as platinum and have a porous chemical coating or the like ontheir surfaces. The electrodes 33 and 34 are a pair of oppositeelectrodes and also called sensor electrodes. The inner space surroundedby the solid electrolyte layer 32 is an air chamber 35, and a heater 36is housed in the air chamber 35. The air chamber 35 is also called thereference chamber. The heater 36 has a sufficient heat generatingcapacity to activate the sensor element 31 and heats the entire sensorelement with its generated heat energy. The activation temperature ofthe O₂ sensor 17 is, for example, about 500 to 650° C. The inside of theair chamber 35 is maintained at a prescribed oxygen concentration byintroduction of the air.

In the above sensor element 31, the outer side of the solid electrolytelayer 32 which is near the exhaust side electrode 33 has an exhaust gasatmosphere and the inner side of the solid electrolyte layer 32 which isnear the air side electrode 34 has an air atmosphere, and depending onthe oxygen concentration difference (oxygen partial pressure difference)between them, an electromotive force is generated between the electrodes33 and 34. In short, an electromotive force which differs depending onwhether the air-fuel ratio is rich or lean is generated. In this case,the exhaust side electrode 33 is lower in oxygen concentration than theair side electrode 34 as the reference electrode and in the sensorelement 31, an electromotive force is generated with the air sideelectrode 34 as the positive side and the exhaust side electrode 33 asthe negative side. Consequently, the O₂ sensor 17 outputs anelectromotive force signal which depends on the oxygen concentration ofthe exhaust gas (namely, air-fuel ratio).

FIG. 3 is an electromotive force characteristic graph which shows therelation between excess air ratio λ of the exhaust gas and theelectromotive force of the sensor element 31. In FIG. 3, the horizontalaxis represents excess air ratio λ and when λ is 1, the air-fuel ratioof the exhaust gas is stoichiometric. The sensor element 31 generates anelectromotive force which differs depending on whether the air-fuelratio is rich or lean, and has a characteristic that the electromotiveforce suddenly changes when the ratio is nearly stoichiometric.Specifically, when the ratio is rich, the electromotive force of thesensor element 31 is about 0.9 V and when the ratio is lean, theelectromotive force of the sensor element 31 is about 0 V. As shown inFIG. 3, an electromotive force characteristic (an output characteristicof the O₂ sensor 17) includes a rapidly-changing voltage region wherethe electromotive force changes rapidly in the vicinity of thestoichiometric point, and stable voltage regions located on both sidesof the rapidly-changing voltage region. In the stable voltage regions,the electromotive force is almost constant.

In FIG. 2, a sensor control section 40 is connected to the sensorelement 31 and when an electromotive force is generated in the sensorelement 31 depending on the air-fuel ratio (oxygen concentration) of theexhaust gas, a sensor detection signal (electromotive force signal)equivalent to the electromotive force is sent to a microcomputer 41 inthe sensor control section 40. The microcomputer 41 calculates theair-fuel ratio according to the electromotive force signal from thesensor element 31. The sensor control section 40 is located in the ECU25 shown in FIG. 1. In the ECU 25, the microcomputer 41 is provided as acalculating section which has an engine control function and a sensorcontrol function. In this case, the microcomputer 41 calculates theengine rotation speed and the intake air amount according to the resultsof detection by the above various sensors. Alternatively, in the ECU 25,a microcomputer for engine control and a microcomputer for sensorcontrol may be provided separately.

The microcomputer 41 makes a determination about the activity state ofthe sensor element 31 and also controls the heater 36 through a heaterdrive circuit 42 according to the result of the determination.

Furthermore, in this embodiment, in order to change the outputcharacteristic of the O₂ sensor 17, a constant current that isprescribed is supplied between the pair of electrodes 33 and 34 in thesensor element 31. In this configuration, the sensor element 31 performsan oxygen pumping. The sensor element 31 increases the exhaust emissionreduction effect in air-fuel ratio feedback control by changing theoutput characteristic. The principle on which the sensor outputcharacteristic is changed by supplying a constant current is as follows.

As shown in FIG. 4, there are CO, HC, NOx, and O₂ in the vicinity of theexhaust side electrode 33 of the O₂ sensor 17 and in this condition, acurrent is supplied to the sensor element 31 so that oxygen ions movefrom the air side electrode 34 to the exhaust side electrode 33 throughthe solid electrolyte layer 32. Specifically, oxygen pumping isperformed in the sensor element 31. In this case, at the exhaust sideelectrode 33, the oxygens which have moved to the exhaust side electrode33 through the solid electrolyte layer 32 react with CO and HC andgenerate CO₂ and H2O. Consequently, CO and HC are removed in thevicinity of the exhaust side electrode 33 and the equilibrium point ofgas reaction in the vicinity of the exhaust side electrode 33 of the O₂sensor 17 shifts to the rich side. In other words, as shown in FIG. 5,the sensor output characteristic which indicates the relation betweenexcess air ratio λ and electromotive force as a whole shifts to the richside and accordingly, the point at which the electromotive force becomesthe stoichiometric value (0.45 V) shifts to the rich side.

Next, the structure of the sensor control section 40 which performscontrol for the O₂ sensor 17 will be described. The structure of thesensor control section 40 is as illustrated in FIG. 2 and the sensorcontrol section 40 has the microcomputer 41 as a control section. Themicrocomputer 41 receives an electromotive force signal from the sensorelement 31 through an A/D converter, etc. and calculates the air-fuelratio of the exhaust gas according to the electromotive force signal.Alternatively, the microcomputer 41 calculates the air-fuel ratio on thecatalyst downstream according to the electromotive force signal. Aconstant current circuit 43 as a constant current supplying section isconnected midway in an electric pathway which electrically connects theair side electrode 34 of the sensor element 31 and the microcomputer 41.When the sensor element 31 generates an electromotive force, theconstant current circuit 43 receives the electromotive force from thesensor element 31 and supplies a current, which depends on theelectromotive force, to the sensor element 31. In this case, accordingto the constant current circuit 43, the current flows from the exhaustside electrode 33 to the air side electrode 34 through the solidelectrolyte layer 32 and accordingly oxygen ions move in the solidelectrolyte layer 32 from the air side electrode 34 to the exhaust sideelectrode 33.

The structure of the constant current circuit 43 of the sensor controlsection 40 and the peripheral circuit around the circuit 43 will bedescribed in more detail referring to FIG. 6.

In FIG. 6, the constant current circuit 43 includes a voltage generatingsection 51 to generate a prescribed constant voltage, an operationalamplifier 52, an n-channel MOSFET 53 to be driven by output of theoperational amplifier 52, and a resistance 54 connected to the source ofthe MOSFET 53. In the voltage generating section 51, a constant voltagesource 51 a and resistances 51 b and 51 c are connected in series andthe middle point between the resistances 51 b and 51 c is voltage outputpoint X1. In this embodiment, the constant voltage source 51 a is 5 V.In the operational amplifier 52, the + input terminal is connected tovoltage output point X1 and the output terminal is connected to the gateof the MOSFET 53. Also, the − input terminal is connected to middlepoint X2 between the MOSFET 53 and the resistance 54. From the viewpointof the MOSFET 53, the gate is connected to the output terminal of theoperational amplifier 52, the drain is connected to the air sideelectrode 34 of the sensor element 31 and the source is connected to theresistance 54.

The above constant current circuit 43 operates so that the voltage ofthe + input terminal of the operational amplifier 52 is equal to thevoltage of its − input terminal, so the voltage at X2 becomes equal tothe voltage at X1. Then, constant current Ics, the amount of which isdetermined by the voltage at X2 and the resistance value of theresistance 54, flows in the series circuit including the sensor element31, MOSFET 53, and resistance 54. At this time, the MOSFET 53 operatesaccording to the operational amplifier output voltage based on thedifference between + and − input voltages and functions as a currentcontrol element which supplies a constant current Ics.

Here, the voltage at X1 and X2 and the resistance value of theresistance 54 should be determined according to the amount of currentwhich is required to flow in the sensor element 31 when an electromotiveforce is generated in the sensor element 31. Specifically, when anelectromotive force (0 to 0.9 V) is generated in the sensor element 31,when a current of 0.1 mA is to flow in the sensor element 31, forexample, the voltage at X1 and X2 should be 10 mV and the resistancevalue of the resistance 54 should be 100Ω. When a current of 0.2 mA isto flow, for example, the voltage at X1 and X2 should be 20 mV and theresistance value of the resistance 54 should be 100Ω. When the currentamount range is to be 0.1 to 2.0 mA, when the resistance value of theresistance 54 is 100Ω, the voltage at X1 and X2 should be in the rage of10 mV to 200 mV.

In the sensor control section 40 which uses the above constant currentcircuit 43, when an electromotive force is generated in the sensorelement 31, the prescribed constant current Ics flows in the MOSFET 53and resistance 54 with the electromotive force as a power source(namely, the sensor element 31 functions as a battery). The outputcharacteristic of the O₂ sensor 17 can be thus changed.

In this embodiment, the constant current Ics supplied by the constantcurrent circuit 43 can be changed according to a command from themicrocomputer 41 and the constant current Ics can be increased ordecreased according to each condition. Specifically, the voltage valueat points X1 and X2 are changed, for example, by changing the resistanceratio between the resistances 51 b and 51 c according to a command fromthe microcomputer 41 and accordingly the constant current Ics ischanged.

The first end of a shunt resistance 55 for current detection isconnected to the exhaust side electrode 33 of the sensor element 31 andthe second end of the shunt resistance 55 is connected to a voltagecircuit 57. The current which flows in the shunt resistance 55 isdetected by a current detecting section 56 and the detection signals aresent to the microcomputer 41 sequentially. The current detecting section56 may be a differential amplifier circuit which uses, for example, anoperational amplifier or the like. In FIG. 2, in the sensor controlsection 40, components such as the shunt resistance 55 and voltagecircuit 57 (other components than the constant current circuit 43 andheater drive circuit 42) are omitted.

The voltage circuit 57, which is intended to apply a positive voltage tothe exhaust side electrode 33, is an offset voltage circuit which makesthe potential of the exhaust side electrode 33 higher by a givenpotential than the potential on the side from which a current flows inthe constant current circuit 43 (grounding side potential of theresistance 54). The voltage circuit 57 has a voltage dividing circuitwhich generates a prescribed offset voltage and the middle point of thevoltage dividing circuit is offset voltage point X3. The voltage at theoffset voltage point X3 is, for example, 2.0 V.

A voltage switch circuit 59 is connected to the air side electrode 34 ofthe sensor element 31. This voltage switch circuit 59 temporarily sweepsthe voltage applied to the sensor element 31 according to a command fromthe microcomputer 41 and the resistance value of the sensor element 31can be detected by the current detecting section 56 detecting the amountof current change with the voltage change. The resistance value of thesensor element 31 is also called the element resistance. The elementresistance is detected in a given cycle and during the detection, thesensor applied voltage is changed by sweeping. When the applied voltageis changed by sweeping, the sensor applied voltage may be changed towardthe positive side or toward both the positive and negative sides. Incalculation of the element resistance, instead of changing the voltageby sweeping, the current may be changed by sweeping so that the elementresistance is calculated from the amount of the resulting voltagechange.

Furthermore, in the sensor control section 40, the heater drive circuit42 has a switching element 42 a which turns on/off the power to theheater 36. In the sensor element 31, heater energization is controlledby turning on/off the switching element 42 a, so that the sensor element31 is maintained in a prescribed active state. In this prescribed activestate, the activation temperature is 500 to 650° C. The control ofheater energization by the microcomputer 41 is briefly outlined below.Before activation of the sensor element 31, in order to expediteactivation, the switching element 42 a is kept ON and the heater 36 isheated with the maximum electric power. In this case, energizationcontrol is wholly performed. After activation of the sensor element 31,the amount of heater energization is feedback-controlled according tothe difference between the target value and the actual value (calculatedvalue) of the element resistance. For example, the amount of dutycontrol at each time is calculated by the PID control method andenergization of the heater is performed by turning on/off the switchingelement 42 a according to the amount of duty control.

In air-fuel ratio control in this embodiment, determination is made atleast as to whether the air-fuel ratio (the air-fuel ratio on thecatalyst downstream) is rich, lean or stoichiometric, on the basis ofcomparison between the electromotive force output of the sensor element31 and a prescribed threshold. Specifically, as thresholds to determinewhether the air-fuel ratio is rich or lean, a first threshold V1 whichis on a richer side than the stoichiometric value for the electromotiveforce of the sensor element 31 and a second threshold V2 which is on aleaner side than the stoichiometric value are determined and when theelectromotive force output is larger than the first threshold V1, themicrocomputer 41 determines that the air-fuel ratio is rich, or when theelectromotive force output is smaller than the second threshold V2, themicrocomputer 41 determines that the air-fuel ratio is lean. The firstthreshold V1 is, for example, 0.6 V and the second threshold V2 is, forexample, 0.3 V. The microcomputer 41 controls the air-fuel ratio on thecatalyst downstream so that the air-fuel ratio is within thenear-stoichiometric range defined by these thresholds V1 and V2.

Next, the relation between electromotive force output and the supply ofconstant current in the above air-fuel ratio determination will bedescribed referring to FIG. 7. In FIG. 7, regarding the outputcharacteristic of the O₂ sensor 17, L1 denotes an output characteristicwithout supply of constant current Ics and L2 and L3 denote outputcharacteristics with supply of constant current Ics. Furthermore, whenthe temperature of the sensor element 31 is low during the cold start ofthe engine 10 or during fuel cut, the element resistance increases andthe value of the electromotive force changes to a negative one due tothe change in the sensor applied voltage with the increase in theelement resistance. In addition to this point, regarding the outputcharacteristics L2 and L3 with supply of constant current Ics, L2indicates an output characteristic (ordinary characteristic) of the O₂sensor 17 in which the resistance has not increased and L3 indicates anoutput characteristic of the O₂ sensor 17 in which the resistance hasincreased. For the convenience of explanation, output characteristiclines are indicated linearly in FIG. 7.

In output characteristic L2, a characteristic change equivalent to theconstant current Ics and element resistance occurs as compared withoutput characteristic L1 (characteristic without current). In outputcharacteristic L3, a characteristic change equivalent to an increase inthe element resistance occurs as compared with output characteristic L2(ordinary characteristic). The width of the near-stoichiometric range inwhich the thresholds V1 and V2 are used for determination is W1 foroutput characteristic L1, W2 for output characteristic L2, and W3 foroutput characteristic L3. In this case, whereas the width of thenear-stoichiometric range for determination in output characteristic L2is nearly equal to that in output characteristic L1 as the basiccharacteristic, the near-stoichiometric range for determination inoutput characteristic L3 is wider than in output characteristic L2 (soto speak, the range of variation is wider). L3 may be considered toindicate that due to an unintentional change in the element resistance,the amount of voltage change is larger than expected, and under suchcondition, the accuracy of air-fuel ratio determination is decreased.Since FIG. 7 shows a case that the output characteristic shifts to therich side, the output characteristic has a voltage change in thenegative direction; on the other hand, when the output characteristicshifts to the lean side, the voltage changes in the positive direction.

As mentioned above, the output characteristic of the O₂ sensor 17 may besaid to have, near the stoichiometric point, a rapidly-changing voltageregion where the electromotive force changes rapidly and, on both sidesof that region, stable voltage regions where the electromotive force isalmost constant (see FIG. 3). Whereas both rich/lean determinations aremade in the rapidly-changing voltage region in output characteristics L1and L2, in output characteristic L3 one of the rich/lean determinationsis made in the rapidly-changing voltage region and the otherdetermination is made in the stable voltage region.

When, while the constant current Ics is supplied to the sensor element31 as mentioned above, the element resistance increases unintentionallyas the temperature of the sensor element 31 becomes low, the air-fuelratio determined as rich (or lean) varies widely and as a consequencethe accuracy of air-fuel ratio determination decreases.

Therefore, in this embodiment, the element resistance Ra is calculatedsuccessively and the constant current Ics to be supplied to the sensorelement 31 is controlled (corrected) according to the element resistanceRa, thereby suppressing the decrease in the accuracy of air-fuel ratiodetermination.

FIG. 8 is a flowchart which shows the constant current control processand this process is repeated by the microcomputer 41 in a given cycle.

In FIG. 8, at S11 the microcomputer 41 determines whether or not theconstant current is now being supplied by the constant current circuit43. At S12, the microcomputer 41 determines whether or not cold start ofthe engine 10 or fuel cut is being executed. In this embodiment, thestep S12 corresponds to a low temperature determining section. When NOat S11 or S12, the microcomputer 41 ends this process or when YES atboth S11 and S12, the microcomputer 41 proceeds to the next step S13.

At S13, the microcomputer 41 acquires the constant current Ics andelement resistance Ra at a present time point. The constant current Icsmay be switched to any one among a plurality of values (for example, 0.1mA, 0.2 mA and so on). For example, the constant current Ics is set as avariable depending on the engine operation condition, etc. In short,when the engine operation condition changes, the amount of richcomponents in the exhaust gas changes accordingly. Specifically, whenthe engine rotation speed is higher or the engine load is larger, theamount of rich components in the exhaust gas increases. In this case, inorder to maintain the desired performance concerning exhaust emissions,it is desirable to control the current to be supplied to the sensorelement 31 (constant current Ics of the constant current circuit 43) asa variable depending on the engine operation condition. For example,when the engine rotation speed is higher or the engine load is larger,the constant current Ics is increased. In this embodiment, the step S13corresponds to a constant current setting section.

The element resistance Ra should be calculated by the microcomputer 41in a given cycle; for example, the element resistance Ra is calculatedthrough the element resistance calculating process shown in FIG. 9. InFIG. 9, at S21 the microcomputer 41 determines whether or not it is timeto calculate the element resistance. When the microcomputer 41determines that it is time to calculate, the microcomputer 41 proceedsto S22. The element resistance calculation interval is, for example, 128msec. At S22, the microcomputer 41 temporarily switches the sensorapplied voltage through the voltage switch circuit 59. At S23, themicrocomputer 41 calculates the amount of current change which occursdepending on the voltage change. Furthermore, at S24 the microcomputer41 calculates the element resistance Ra from the amount of currentchange calculated at S23. In this embodiment, the step S24 correspondsto a resistance value calculating section.

At S14, the microcomputer 41 sets a current correction value Ki tocorrect (decrease) the constant current Ics, according to the elementresistance Ra. At this time, the current correction value Ki is set, forexample, using the relation in FIG. 10 (a). According to FIG. 10 (a),when the element resistance Ra is A1 or more, the current correctionvalue Ki is set to a larger value for a larger element resistance Ra. Inother words, when the element resistance Ra is less than A1, theconstant current Ics is not corrected (decreased) but when the elementresistance Ra is A1 or more, the constant current Ics is corrected(decreased). A1 should be a target element resistance in heaterenergization control or a resistance value which is nearly equal to thetarget element resistance. In the arrangement that the constant currentIcs is set as a variable, as shown in FIG. (b) the current correctionvalue Ki should be set according to the element resistance Ra andconstant current Ics. According to FIG. 10 (b), the element resistanceRa as the reference to determine whether to correct (decrease) theconstant current Ics is set to a value which differs depending on theconstant current Ics, namely B1, B2, or B3, and when the constantcurrent Ics is larger, the element resistance Ra as the reference issmaller. As shown in FIG. 10 (b), B1 is smaller than B2 and B2 issmaller than B3. When the element resistance Ra is the same, the currentcorrection value Ki is set to a larger value for a larger constantcurrent Ics.

The control of constant current Ics should be performed so that in thesensor output characteristic having a rapidly-changing voltage regionand a stable voltage region, the thresholds V1 and V2 remain included inthe rapidly-changing voltage region. In other words, rich determinationand lean determination are always made in the rapidly-changing voltageregion of the sensor output characteristic. In this case, the set valueof constant current Ics itself is determined within such a range thatthe thresholds V1 and V2 are included in the rapidly-changing voltageregion, and the condition in which the thresholds V1 and V2 are includedin the rapidly-changing voltage region can be maintained by performingcurrent control so that a voltage change corresponding to Ics occurs.

After that, at S15 the microcomputer 41 corrects a present constantcurrent Ics that is the constant current Ics at the present time point,by the current correction value Ki calculated at S14. In thisembodiment, the steps S14 and S15 correspond to a current controlsection and also the step S14 corresponds to a correction valuecalculating section and the step S15 corresponds to a correctionsection. Specifically, as expressed by the formula (1) below, thecorrected constant current Ics is the present constant current Ics minusthe current correction value Ki. After the correction, the constantcurrent Ics supplied by the constant current circuit 43 is controlledaccording to the corrected constant current Ics.

Ics=Ics−Ki  (1)

According to the embodiment detailed above, the following advantageouseffects can be brought about.

Since the constant current Ics supplied to the sensor element 31 iscontrolled according to the element resistance Ra, the disadvantage thatthe accuracy of air-fuel ratio determination decreases unintentionallycan be suppressed. In other words, the increase in the amount of changein the sensor applied voltage due to increase in the element resistanceRa can be suppressed by decreasing the constant current Ics. Thus, thedecrease in the accuracy of air-fuel ratio determination can besuppressed. Consequently, air-fuel ratio determination can be madeappropriately while the constant current Ics is supplied to the O₂sensor 17.

One method of suppressing the increase in the amount of change in thesensor applied voltage other than by decreasing the constant current Icsis to decrease the element resistance by increasing the amount of heatgeneration by the heater 36. However, more electric power is required toincrease the amount of heat generation by the heater 36, which isdisadvantageous in terms of energy saving. In addition, when the elementresistance is decreased by heating by the heater 36, a delay occurs fromwhen heating is started until the temperature of the sensor element 31actually increases, which may be undesirable from the viewpoint ofresponse speed (element resistance restoration speed). Furthermore, anovershoot, etc. may occur when the element resistance changes, which maybe undesirable from the viewpoint of controllability.

The current correction value Ki is calculated according to the elementresistance Ra and the constant current Ics is corrected by the currentcorrection value Ki (see FIG. 10 (a)). Therefore, even when the elementresistance Ra changes unintentionally, the constant current Ics can becontrolled appropriately in accordance with the change.

The current correction value Ki is calculated according to the elementresistance Ra and constant current Ics (value set as a variable) and theconstant current Ics is corrected by the current correction value Ki(see FIG. 10 (b)). In the arrangement that the constant current Ics isset as a variable, even when the element resistance Ra changesunintentionally, the constant current Ics can be controlledappropriately in accordance with the change.

The constant current is controlled so that in the electromotive forcecharacteristic of the O₂ sensor 17 having a rapidly-changing voltageregion and a stable voltage region, rich determination and leandetermination are made in the rapidly-changing voltage region.Consequently, variance in rich determination and lean determination canbe suppressed with certainty.

During the cold start of the engine 10 or during fuel cut, thetemperature of the sensor element 31 is relatively low. At such a lowtemperature, wrong air-fuel ratio determination is likely to occur. Inthis respect, when it is determined that cold start or fuel cut is beingexecuted, the above constant current control is performed, so acondition which is likely to cause a disadvantage can be addressedproperly.

OTHER EMBODIMENTS

The above embodiment may be altered as follows.

(a) Under the condition that a constant current is supplied to thesensor element 31, when feedback control of the heater 36 is performedso as to control the element resistance Ra to be a target value Rtg, theconstant current Ics may be changed when the element resistance Ra isdifferent from the target value Rtg by a prescribed amount or more.

FIG. 11 is a flowchart which shows the constant current control processand this process is repeated by the microcomputer 41 in a given cycle.In FIG. 11, at S31 the microcomputer 41 determines whether or not thesupply of constant current by the constant current circuit 43 is beingexecuted. At S32, the microcomputer 41 determines whether or notfeedback control of the heater 36 is being executed. In this embodiment,the step S32 corresponds to a heater control section. When NO at S31 orS32, the microcomputer 41 ends this process or when YES at both S31 andS32, the microcomputer 41 proceeds to the next step S33.

At S33, the microcomputer 41 acquires the target value Rtg and elementresistance Ra. At S34, the microcomputer 41 sets a current correctionvalue Ki to correct (decrease) the constant current Ics, according todeviation ΔR which is the difference between the target value Rtg andthe element resistance Ra (=Ra−Rtg). At this time, the currentcorrection value Ki is set using the relation in FIG. 12. The deviationΔR is the element resistance Ra minus the target value Rtg as expressedby the formula (2) below.

ΔR=Ra−Rtg  (2)

According to FIG. 12, when the element resistance deviation ΔR is C1 orlarger, the current correction value Ki is set to a larger value for alarger deviation ΔR. In other words, when the deviation ΔR is smallerthan C1, the constant current Ics is not corrected (decreased) but whenthe deviation ΔR is C1 or larger, the constant current Ics is corrected(decreased).

After that, at S35 the microcomputer 41 corrects the present constantcurrent Ics by the current correction value Ki calculated at S34. Inthis embodiment, the steps S34 and S35 correspond to a current controlsection, the step S34 corresponds to a correction value calculatingsection and the step S35 corresponds to a correction section.Specifically the present constant current Ics is corrected in accordancewith the above formula (1). After the correction, the constant currentIcs supplied by the constant current circuit 43 is controlled accordingto the corrected constant current Ics.

According to the above arrangement, even when a deviation of the elementresistance Ra from the target value Rtg occurs with a sudden change inthe exhaust gas temperature, etc., air-fuel determination can be madeappropriately.

(b) In the above embodiment, two thresholds V1 and V2 are used for therich side and the lean side for the purpose of air-fuel ratiodetermination; however, instead of this, only one of them may be used.For example, only the first threshold V1 for the rich side may be usedto determine whether or not the air-fuel ratio is rich.(c) In the above embodiment, an example in which the present disclosureis applied to an O₂ sensor 17 with a heater has been explained; however,instead of this, the disclosure may be applied to an O₂ sensor without aheater. In this case as well, a condition in which the elementtemperature decreases (element resistance changes) can be addressedappropriately as mentioned above.(d) In the above embodiment, determination is made as to whether coldstart of the engine 10 or fuel cut is being executed and when it isdetermined that cold start or fuel cut is being executed, constantcurrent control is performed; instead, however, constant current controlmay be performed regardless of whether or not cold start or fuel cut isbeing executed. Namely, S12 in FIG. 8 may be omitted.(e) For example, when the exhaust gas temperature rises with high loadoperation of the engine 10 and the element resistance Ra decreases, thesensor applied voltage also changes due to the change in the resistance.In such case, the constant current Ics should be increased.(f) The structure of the constant current supplying section is notlimited to the above constant current circuit 43 but any structure thatcan supply a prescribed constant current and vary the value of thecurrent may be adopted. For example, a constant current circuit whichcan adjust the amount of current by PWM control (duty control) may beused. When that is the case, the constant current may be adjusted as avariable according to a current restriction command.(g) In the above embodiment, the O₂ sensor 17 is located downstream ofthe first catalyst 15 a; instead, however, the O₂ sensor 17 may belocated in the middle portion of the first catalyst 15 a. In this case,the O₂ sensor 17 may be located on the support of the first catalyst 15a. In any case, the O₂ sensor 17 has only to take the exhaust gaspurified by the first catalyst 15 a as the object of detection anddetect the gas components.(h) The gas sensor is not limited to the above O₂ sensor 17, but insteadthe gas sensor may be a so-called 2-cell gas sensor which includes anelectrogenic cell and a pump cell. In this case, the outputcharacteristic of the electrogenic cell of the 2-cell gas sensor can bechanged properly and air-fuel ratio determination can be madeappropriately.

While the present disclosure has been described with reference toembodiments thereof, it is to be understood that the disclosure is notlimited to the embodiments and constructions. The present disclosure isintended to cover various modification and equivalent arrangements. Inaddition, while the various combinations and configurations, othercombinations and configurations, including more, less or only a singleelement, are also within the spirit and scope of the present disclosure.

1. A gas sensor control device for a gas sensor which has anelectrogenic cell using a solid electrolyte body and a pair ofelectrodes placed at a position to interpose the solid electrolyte body,and detects an exhaust gas from an internal combustion engine as anobject of a detection and outputs an electromotive force signaldepending on an air-fuel ratio of the exhaust gas, the gas sensorcontrol device comprising: a constant current supplying sectionsupplying a constant current that is prescribed to the electrogeniccell; a resistance value calculating section calculating a resistancevalue of the electrogenic cell; an air-fuel ratio determining sectiondetermining whether the air-fuel ratio is at least rich, lean, orstoichiometric, on the basis of a comparison between an electromotiveforce output of the electrogenic cell and a prescribed threshold; and acurrent control section controlling the constant current supplied by theconstant current supplying section, on the basis of the resistance valueof the electrogenic cell calculated by the resistance value calculatingsection.
 2. The gas sensor control device according to claim 1, whereinthe current control section includes a correction value calculatingsection calculating a current correction value for the constant currentbeing supplied at a present time point, on the basis of the resistancevalue of the electrogenic cell calculated by the resistance valuecalculating section, and a correction section correcting a presentconstant current that is the constant current at the present time point,by the current correction value.
 3. The gas sensor control deviceaccording to claim 1, further comprising: a constant current settingsection setting the constant current supplied by the constant currentsupplying section as a variable, wherein the current control sectionincludes a correction value calculating section calculating a currentcorrection value for the present constant current on the basis of arelation between the resistance value of the electrogenic cellcalculated by the resistance value calculating section and the presentconstant current set by the constant current setting section, and acorrection section correcting the present constant current by thecurrent correction value.
 4. The gas sensor control device according toclaim 1, wherein the electrogenic cell has an output characteristicincluding a rapid change region where the electromotive force outputchanges rapidly near a stoichiometric point and a stable region on aricher or leaner side than the rapid change region where theelectromotive force output is almost constant, and the current controlsection controls the constant current so that the prescribed thresholdremains included in the rapid change region of the outputcharacteristic.
 5. The gas sensor control device according to claim 1 4,further comprising: a low temperature determining section determiningthat a cold start of the internal combustion engine or a fuel cut isbeing executed, wherein when the low temperature determining sectiondetermines that the cold start or the fuel cut is being executed, thecurrent control section performs a control of the constant current. 6.The gas sensor control device according to claim 1, the gas sensorcontrol device for the gas sensor provided with a heater heating theelectrogenic cell, the gas sensor control device further comprising: aheater control section controlling a drive of the heater so as tocontrol the resistance value of the electrogenic cell calculated by theresistance value calculating section to be a target resistance valueunder a condition that the constant current is supplied by the constantcurrent supplying section, wherein the current control section changesthe constant current when the resistance value of the electrogenic celldeviates from the target resistance value by a prescribed amount or moreduring a heater control performed by the heater control section.